Microwave spectrum and substitution structure of syn thiobenzoic acid

The rotational spectrum of the syn conformer of thiobenzoic acid (C₆H₅COSH) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectra of all singly-substituted isotopologues involving ¹³C, as well as the ¹⁸O, ³³S, ³⁴S, and −SD derivatives have also been recorded. The isotopic data have been used to derive structural parameters of the molecular frame. The results are compared with density functional theory calculations at theM06-2X/6-311++G(d,p) level and show excellent agreement. The molecule is planar at the equilibrium geometry, and calculations of the energy profile for out-of-plane torsion of the SH group are presented. The anti conformer was not observed, consistent with calculations which indicate that it lies 2.6 kcal/mol higher in energy than the syn form.